I have an outbuilding which is a single-skinned extension to the garage. The outbuilding and garage are about 40cm below the level of the main building. The outbuilding is separated from the main building by a sunken patio and a retaining wall (see photos). Previously the patio did not extend the full length of the building, which was partially buried, and there was no proper drainage from the patio so water would build up whenever it rained. This also led to damp issues in the outbuilding. I recently had builders extend the patio along the whole length of the outbuilding and create a channel along the side, the idea being that the water should be able to drain along the channel onto the driveway in front of the garage (which has permeable paving). However, the builders didn't create a proper fall to the channel so it slopes inwards toward the patio, and the patio itself is angled like a bowl so that water collects in front of the doorway. Obviously the builder has not done a proper job but I would like some advice as to what is the best way to create drainage here?
The additional problem is that the top of the patio slabs are higher than the bricks under the door (which have not been damp proofed), so damp collects on these bricks - not sure if this can penetrate into the building (it is single skinned and there is a ventilation gap behind, so some damp should be manageable but I'm concerned the patio is too high relative to the building walls.
1) Raise the patio slabs a couple of cm and apply a skim of cement to the channel to create the necessary fall allowing water to drain as originally planned? The problem with this is that it requires raising the patio further so that the top of it is very close to the bottom of the doorway, and results in the bricks under the door becoming completely buried - or is this not an issue if they are covered with damp proof render/cement?
2) Excavate the channel along the wall - this will require removing the retaining wall along the channel as the bottom of the channel is made of thick concrete paver slabs which also serve as the foundation for the retaining wall (I think this is pretty poor workmanship, but I am no expert). The builder is refusing to do this.
3) Create a soakaway for the water to drain somewhere else - the problem is that the patio sits on top of a concrete slab which is contiguous with the outbuilding foundation, so water can't penetrate downward and the only location for a soakaway would be under other paving adjacent to the house, not only would this require a lot of building work and excavation, it is also very close to the main building so not suitable location for a soakaway. The soil is also pretty heavy so not convinced it would work anyway.
Most of the water from the outbuilding roof goes into a separate water tank, so normally there won't be massive amounts of rainwater, but any larger than average rainfall results in a big pond of water right up to the wall of the outbuilding.
The reason some of the slabs have been lifted in the photos is that I wanted to dig underneath to see if I could provide drainage that way.
Hope any of this makes sense, any advice would be appreciated.
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